Subterranean formations that include coal seams can contain substantial quantities of methane gas. Extracting this gas may help protect mining personnel from dangerous exposures to methane and may allow the producer to derive profit from sale of the gas as an energy source. While conventional reservoirs store methane as a free gas under pressure, coal's unique structure allows it also to store gas through adsorption onto its surface. The gas adsorbs into micropores that dot the surface of coal. The high density of these micropores yields 10 to 100 square meters of surface area per gram of coal, giving coal beds the capacity to adsorb significant amounts of gas. The amount of gas a particular coal bed can store depends on the interplay of several factors other than its structural properties, such as the temperature and pressure of the reservoir, the composition of the coal, and the composition and molecular properties of the gas.
Generally, the closer wells are spaced, the greater gas recovery may be over the economic life of the wells. Wells are ideally spaced to maximize gas liberation by minimizing the reservoir pressure in the coal seam across a large area. Because coal stores gas by adsorption, producers must depressurize coal beds to desorb the gas from the coal to begin gas flow. The lower reservoir pressure allows the gas to diffuse out of the coal. A reduction in reservoir pressure can be achieved by spacing many wells in close proximity, with the actual distance between each well determined by the permeability of the coal seam, among other factors. The production of gas by one well will reduce the pressure in the reservoir and affect production by neighboring wells. This well “interference” is determined by a number of factors, including, but not limited to, permeability, permeability anisotropy and well spacing. By spacing wells to maximize interference, coal beds can be rapidly depressurized to stimulate gas flow. Wells are spaced to yield maximum interference within four to six years to allow for maximum production within an economic time frame. Because subterranean water often accompanies methane gas in coal seams, reservoir pressure can also be reduced by removing this water while preventing localized water recharge. The less distance a water or gas molecule must travel to a well, the greater production will be within the economic time frame of the wells. Well spacing is therefore a critical design element in any gas production system.
Horizontal wells allow for close well spacing without the high cost and negative environmental impact of drilling closely spaced vertical wells. Members of the mining industry have produced methane from coal beds for years with horizontal wells rather than with vertical wells alone. Horizontal wells used to produce methane have been drilled as branches of well bores that are initiated at the depth of the coal seam from vertical mine shafts or well bores. The horizontal wells have been drilled in patterns that require drilling extensive networks of horizontal well bores, such as pinnate patterns. However, while drilling extensive networks of horizontal well bores in coal seams may be cheaper than drilling a multitude of closely spaced vertical wells alone, doing so may still be economically infeasible. The more drilling a project requires, the more costly and time-consuming it will be. Although horizontal wells of the prior art reduced the number of vertical wells needed to extract methane from coal beds, the prior art still requires drilling multiple vertical wells. Depending on the characteristics of the formation, seam and site, these vertical wells may need to be spread over entire the surface of a coal seam if prior art patterns are used.
Well bore instability limits the applicability of prior art horizontal well systems more than the characteristics of a particular reservoir. Horizontal well systems of the prior art have provided for open hole operation. That is, the horizontal well bores have lacked linings or casings. Collapses of unlined and uncased well bores in coal seams are quite common, as coal seams often do not have the strength to withstand extensive open hole operations. Fracturing open hole well bores in coal seams can be particularly hazardous. Linings and casings increase well bore stability and thus are generally desirable. However, as with drilling, lining or casing extensive networks of horizontal wells can be prohibitively expensive.